The present invention concerns solar cells, particularly regenerative solar cells, light harvesting arrays useful in such solar cells, light harvesting rods for use therein, methods of making light-harvesting rods, and intermediates useful for the manufacture of light-harvesting rods.
Molecular approaches for converting sunlight to electrical energy have a rich history with measurable xe2x80x9cphotoeffectsxe2x80x9d reported as early as 1887 in Vienna (Moser, J. Montash. Chem. 1887, 8, 373.). The most promising designs were explored in considerable detail in the 1970""s (Gerischer, H. Photochem. Photobiol. 1972, 16, 243; Gerischer, H. Pure Appl. Chem. 1980, 52, 2649; Gerischer, H.; Willig, F. Top. Curr. Chem. 1976, 61, 31). Two common approaches incorporate molecules that selectively absorb sunlight, termed photosensitizers or simply sensitizers, covalently bound to conductive electrodes. Light absorption by the sensitizer creates an excited state, that injects an electron into the electrode and then oxidizes a species in solution. Such a photoelectrosynthetic cell produces both electrical power and chemical products. Many of the molecular approaches over the past few decades were designed to operate in the manner shown with the goal of splitting water into hydrogen and oxygen. In contrast, a regenerative cell converts light into electricity with no net chemistry. In the regenerative solar cell, the oxidation reactions that take place at the photoanode are reversed at the dark cathode.
The principal difficulty with these solar cell designs is that a monolayer of a molecular sensitizer on a flat surface does not absorb a significant fraction of incident visible light. As a consequence, even if the quantum yields of electron transfer are high on an absorbed photon basis, the solar conversion efficiency will be impractically low because so little light is absorbed. Early researchers recognized this problem and tried to circumvent it by utilizing thick films of sensitizers. This strategy of employing thick absorbing layers was unsuccessful as intermolecular excited-state quenching in the thick sensitizer film decreased the yield of electron injection into the electrode.
A number of additional approaches have been taken. One class of thick film sensitizers is provided by the so-called organic solar cells (Tang, C. W. and Albrecht, A. C. J. Chem. Phys. 1975, 63, 953-961). The state-of-the-art organic solar cells are multilayer organic xe2x80x9cheterojunctionxe2x80x9d films or doped organic layers that yield xcx9c2% efficiencies under low irradiance, but the efficiency drops markedly as the irradiance approaches that of one sun (Forrest, S. R. et al., J. Appl. Phys. 1989, 183, 307; Schon, J. H. et al., Nature 2000, 403, 408). Another class of molecular-based solar cells are the so-called photogalvanic cells that were the hallmark molecular level solar energy conversion devices of the 1940""s-1950""s (Albery, W. J. Acc. Chem. Res. 1982, 15, 142). However, efficiencies realized to date are typically less than 2%.
In 1991, a breakthrough was reported by Gratzel and O""Regan (O""Regan, B. et al., J. Phys. Chem. 1990, 94, 8720; O""Regan, B. and Grxc3xa4tzel, M. Nature 1991, 353, 737). By replacing the planar electrodes with a thick porous colloidal semiconductor film, the surface area for sensitizer binding increased by over 1000-fold. Gratzel and O""Regan demonstrated that a monolayer of sensitizer coating the semiconductor particles resulted in absorption of essentially all of the incident light, and incident photon-to-electron energy conversion efficiencies were unity at individual wavelengths of light in regenerative solar cells. Furthermore, a global efficiency of xcx9c5% was realized under air-mass 1.5 illumination conditions; this efficiency has risen to a confirmed 10.69% today (Gratzel, M. in xe2x80x9cFuture Generation Photovoltaic Technologiesxe2x80x9d McConnell, R. D.; AIP Conference Proceedings 404, 1997, page 119). These xe2x80x9cGratzelxe2x80x9d solar cells have already found niche markets and are commercially available in Europe.
These high surface area colloidal semiconductor films (Gratzel cells) achieve a high level of absorption but also have the following significant drawbacks. (1) A liquid junction is required for high efficiency (because the highly irregular surface structure makes deposition of a solid-state conductive layer essentially impossible). (2) The colloidal semiconductor films require high temperature annealing steps to reduce internal resistances. Such high temperatures impose severe limitations on the types of conductive substrates that can be used. For example, polymeric substrates that melt below the required annealing temperatures cannot be used. (3) Significant losses are associated with transporting charge through the thick semiconductor films. These losses do not appreciably decrease the photocurrent, but have a large effect on the voltage output and thus the power is decreased significantly (Hagfeldt, A.; Grxc3xa4tzel, M. Chem. Rev. 1995, 95, 49). Accordingly, there remains a need for new molecular approaches to the construction of solar cells.
A first aspect of the present invention is a light harvesting array, comprising:
(a) a first substrate comprising a first electrode; and
(b) a layer of light harvesting rods electrically coupled to the first electrode, the light harvesting rods comprising, consisting essentially of or consisting of an oligomer of Formula I:
A1(Ab+1)bxe2x80x83xe2x80x83(I)
xe2x80x83wherein:
(i) b is at least 1;
(ii) A1 through Ab+1 are covalently coupled rod segments, which segments are different and which segments have sequentially less positive electrochemical potentials; and
(iii) each segment A1 through A1+b comprises a compound of Formula II:
X1(Xm+1)mxe2x80x83xe2x80x83(II)
xe2x80x83and wherein:
m is at least 1; and
X1 through Xm+1 are covalently coupled porphyrinic macrocycles.
For example, X1 through Xm+1 may be selected from the group consisting of chlorins, bacteriochlorins, and isobacteriochlorins; b may be from 1 to 2, 5 or 10; m may be from 1 or 2 to 5, 10, or 20; in some embodiments, at least one, or all, of X1 through Xm+1 may be meso-linked porphyrinic macrocycles; in other embodiments, at least one, or all, of X1 through Xm+1 may be xcex2-linked porphyrinic macrocycle (and particularly trans xcex2-linked porphyrinic macrocycles). In one embodiment, each porphyrinic macrocycle X1 through Xm+1 is the same within each individual rod segment.
In general, the light harvesting rods are preferably linear, are preferably oriented substantially perpendicularly to the first electrode, and are preferably not greater than 500 nanometers in length. The light harvesting rods are preferably intrinsic rectifiers of excited-state energy, and are preferably intrinsic rectifiers of ground-state holes.
The substrate in the light harvesting array may be rigid or flexible, transparent or opaque, and may be substantially planar in shape. The electrode may comprise a metallic or nonmetallic conductor. Substrates and electrodes in solar cells as described below may be of the same materials as substrates and electrodes in the light harvesting arrays described herein.
A further aspect of the present invention is a solar cell comprising a light harvesting array as described above, and a second substrate comprising a second electrode, with the first and second substrate being positioned to form a space therebetween, and with at least one of (i) the first substrate and the first electrode and (ii) the second substrate and the second electrode being transparent. There is optionally but preferably an electrolyte in the space between the first and second substrates. The electrolyte may be aqueous or nonaqueous, polymeric or nonpolymeric, liquid or solid, etc. In one embodiment, the solar cell is devoid of (i.e., free of) liquid in the space between the first and second substrates. In some embodiments, the light harvesting rods may be electrically coupled to the second electrode. In some embodiments, a mobile charge carrier may be included in the electrolyte.
A further aspect of the present invention is a composition of light harvesting rods, the light harvesting rods comprising, consisting essentially of or consisting of an oligomer of Formula I as described above.
A still further aspect of the present invention is a method of making a composition of light harvesting rods, the light harvesting rods comprising, consisting essentially of or consisting of an oligomer of Formula I as given above. The method comprises the steps of:
(a) providing a first rod segment of Formula III and a second rod segment of Formula IV:
E[X1(Xm+1)m]1fxe2x80x83xe2x80x83(III)
G[X1(Xm+1)m]2Txe2x80x83xe2x80x83(IV)
xe2x80x83wherein:
X1, Xm+1, and m are as given above;
E is an end group;
one of f or G is an ethynyl group, and
the other of f or G is a halo group (preferably not iodo); and
T is an end group; and then
(b) coupling, preferably by Sonogoshira coupling, the segment of Formula III to the segment of Formula IV to produce a compound of Formula I. In a particular embodiment, f is an ethynyl group; and G is a halo group. In a more particular embodiment, E is a bromo group; f is an ethynylphenyl group; G is an iodo group; and T is a protected ethynyl group.
In a further embodiment, E is a halo group (preferably different from whichever of f or G is a halo group) and the providing step (a) further comprises providing a compound of Formula V:
Z[X1(Xm+1)m]3Jxe2x80x83xe2x80x83(V)
wherein X1, Xm+1, and m are as given above, Z is an end group and J is an ethynyl group; the method further comprising the step of: (c) coupling (preferably by Sonogashira coupling) the segment of Formula V to the product of the prior coupling step (b) to produce a compound of Formula I.
A still further aspect of the present invention is a rod segment useful for the production of light harvesting rods, the rod segment comprising a compound of Formula III:
E[X1(Xm+1)m]1fxe2x80x83xe2x80x83(III)
wherein:
E is selected from the group consisting of bromo, chloro, and fluoro (preferably bromo);
f is an ethynyl group, preferably an ethynylphenyl group, which may be protected or unprotected;
m is as given above;
X1 through Xm+1 are covalently coupled porphyrinic macrocycles; and
each porphyrinic macrocycle X1 through Xm+1 is the same.
A variety of different electrical devices comprised of a solar cell as described above having circuits (typically resistive loads) electrically coupled thereto can be produced with the solar cells of the invention.